Circumstantial evidence, consisting of the presence of insulin and insulin binding sites in the mammalian brain and the absence of consistent metabolic effects of insulin analogous to those seen in insulin-receptive non-neural tissues, has led to speculation that insulin has a role as a neurotransmitter in the central nervous system. The studies proposed are designed to investigate the potential roles of insulin as a neurotransmitter and as a neuromodulator which affects the efficiency of the activities of other neurotransmitter. Nerve ending preparations (synaptosomes) from several brain regions of the adult Sprague-Dawley rat will be utilized to focus the experiments on potential presynaptic actions of insulin and to avoid indirect effects of insulin on the brain due to hypoglycemia or to effects on the blood-brain barrier. Evidence is presented that insulin stimulates synaptosomal high-affinity, Na+-dependent uptake of putative neurotransmitters. Since these uptake systems may function in vivo to reduce the synaptic level, and thus the synaptic activity, of neurotransmitters, the increased efficiency of these transport systems in the presence of insulin represents a potential mechanism for neuromodulation. The stimulation of Na+-dependent uptake may be the result of an observed neurotransmitter-like action of insulin on synaptosomal membrane electrical potential. Experiments are designed to define the specificity of the stimulatory effect of insulin on the transport of putative neurotransmitters and to determine the mechanism of this in vitro action. Other experiments will examine the subcellular distribution of insulin and will determine the distribution of insulin binding sites in neuronal and glial elements of a given brain region with respect to the localization of the neurotransmitter transport systems in the region. The results will help to elucidate the role of insulin in the mammalian brain by identifyng biochemical mechanisms for neurotransmitter-like and neuro-modulatory actions of insulin. Understanding these actions of insulin at this level is an essential prelude to the clarification of the specific physiological processes which involve insulin and its receptor system in the central nervous system. For other neurotransmitters, this has been accompanied by an awareness of the association of alterations in neurotransmitter function in nervous tissue with abnormal behavior and disease states. The realization of such an association has facilitated therapeutic advances, in addition to enhancing our understanding of brain function.